Parkinson's disease (PD) is an age-related neurodegenerative disorder affecting approximately 5% of people over age 65. PD is characterized pathologically by the selective degeneration of dopaminergic neurons in the substantia nigra and the formation of intraneuronal inclusions known as Lewy bodies. Recessively inherited mutations in the Parkin gene are the most common cause of inherited and early onset PD. A variety of large Parkin deletion and truncation mutations as well as missense mutations have been linked to PD in many families, strongly indicating that recessively inherited parkinsonism is caused by loss of Parkin function. The central hypothesis underlying our research is that loss-of-function mutations in the Parkin gene alter the normal physiology of dopaminergic neurons in the substantia nigra, ultimately leading to the parkinsonian phenotype. A loss-of-function pathogenic mechanism can be studied in cells and animals from which the Parkin gene has been deleted. Knockout mice are commonly used to investigate the normal function of genes. Knockout mice can also be used to study diseases caused by gene deletions in humans. Parkin knockout mice can be used to study the abnormal nigral degeneration caused by loss of Parkin function in humans. To investigate the role of Parkin in the survival of dopaminergic neurons, we propose to generate mice with targeted germ-line disruption of the Parkin locus. The Parkin knockout mice will then be analyzed for biochemical and neuropathological abnormalities associated with PD, such as degeneration of dopaminergic neurons, reductions in striatal dopamine levels, and motor behavioral deficits. In parallel, we will generate and analyze Parkin knockout cells in vitro. This will provide a powerful cellular system with which to characterize the function of Parkin and to examine the consequences of its absence, such as increased sensitivity to oxidative stress and apoptotic stimuli. Both the animal and the cellular systems could provide valuable means for identifying and testing molecules and genes with therapeutic potential.